It has been recognized that mRNA obtained from plasma of a blood sample can be useful as an indicator of protein expression. Thus, the presence of extra-cellular or cell-free mRNA in blood plasma has triggered a variety of investigations aimed at determining the source and possible diagnostic capabilities of these nucleic acids. Given that the blood of most healthy individuals ordinarily does not contain substantial amounts of cell-free RNA, elevated amounts of cell-free nucleic acids are usually indicative of a health issue (or pregnancy, as fetal cell-free nucleic acids have been identified in maternal blood). Specifically, elevated presence of cell-free mRNA has been found to indicate the existence of various cancers thereby providing support for the belief that these nucleic acids may originate from tumor cells. Consequently, the identification of cell-free RNA within a blood sample could provide insight into the presence and severity of cancer or some other condition (e.g., diabetes, inflammation, arthritis, infection, or the like), and may thus be an early indicator of such condition. The identification of cell-free RNA within a blood sample may also provide guidance for how best to treat a patient depending upon the relative amounts of cell-free nucleic acids identified within the patient's blood sample. Thus it may be useful not only for diagnosis, but also for patient treatment (e.g., by evaluating a change in the RNA condition of a patient as treatment progresses).
RNA is typically subject to ribonuclease (RNase) activity that reduces the amount of recoverable RNA from a blood sample. However, it has been discovered that despite the presence of ample RNase activity in blood plasma, cell-free mRNA is unexpectedly very stable in vivo and avoids any substantial nuclease mediated degradation. However, after a blood sample is acquired from a patient, cell lysis begins and the nucleic acids from within the blood cells are mixed with the cell-free nucleic acids, making it difficult if not impossible to isolate and distinguish the cell-free mRNA. Further, there is concern about the stability of the cell-free nucleic acids and their ability to avoid nuclease-initiated degradation in vitro. Consequently, the disease indication capability of the cell-free nucleic acids may be diminished as their presence is no longer accurately ascertainable. Ideally, prevention of cell lysis and cell-free nucleic acid degradation within the blood sample would allow for the cell-free nucleic acids to be accurately measured and the presence of any disease risk to be detected.
Efforts to further understand the unexpected stability of the cell-free nucleic acids in vivo have led to the belief that these nucleic acids are able to avoid nuclease activity through protection from proteins or by being packaged into apoptotic bodies. In other words, as cells undergo cell death or apoptosis, apoptotic bodies are produced and the cell-free nucleic acids become encased within a membrane of the apoptotic body, thereby reducing the susceptibility of the nucleic acids to nucleases. However, there is concern that after blood draw the nucleic acids somehow become disassociated from the apoptotic bodies and become vulnerable to nucleases. There is a resulting need to process the blood samples containing the cell-free nucleic acids so that the nucleic acids continue to be unaffected by nucleases throughout processing to produce accurate counts of cell-free nucleic acids for diagnosis purposes.
Metabolic inhibitors have been employed previously to inhibit metabolism in cells. For example, glyceraldehyde, sodium fluoride, and ATA, have been used to inhibit glucose metabolism in blood cells. See, e.g., U.S. Pat. Nos. 5,614,391; and 7,390,663 incorporated by reference herein. The use of formaldehyde-donor preservatives for cell or tissue, or RNA preservation has been described in U.S. Pat. Nos. 5,196,182; 5,260,048; 5,459,073; 5,811,099; 5,849,517; and 6,337,189, incorporated by reference herein.
A number of patent documents address processes for the stabilization and/or identification of nucleic acids located within blood plasma and their diagnostic applications. See, generally, U.S. Pat. Nos. 5,614,391; 5,985,572; 6,617,170; 6,630,301; 6,759,217; 6,821,789; 6,916,634; 6,939,671; 6,939,675; 7,208,275; 7,288,380, 7,569,350 and U.S. Patent Publication Nos. 2008/0057502; 2008/0096217; and 2008/0261292 all incorporated by reference herein. Notwithstanding the above, there remains a need for cell-free RNA isolation and preservation methods that preserve cell-free RNA substantially as it exists at the time of a blood draw in an effort to maximize the amount of recovered cell-free nucleic acid from blood plasma and produce reliable isolation and diagnostic results.
The present invention addresses the need for an efficient and consistent method of preserving and testing of a blood sample for elevated levels of cell-free RNA in plasma of the blood sample, which unexpectedly and surprisingly results in short term inhibition of metabolism (i.e., RNA synthesis); long term fixing of blood cells of the blood sample to prevent leaking of cellular RNA into the plasma; fixing the cellular RNA that is within the blood cells to freeze (e.g., immobilize) the protein expression pattern of the blood cells; and stabilizing and protecting the RNA that is in the plasma from nucleases and proteases.